There's no real template other than to apologize and perhaps let him know how you might make up for it.

While you wouldn't make excuses, an explanation might soften the offense or disappointment you caused.

Thank you Lad. I prayed to Our Lord and prostrated before a statue of Christ before sending an apology text to ask for my employer's pardon.
After he sent me home prematurely (for good reason) I saw/heard a lot of religious symbols on the commute home (a truck with an image of the three crosses of Calvary on it and a verse from scripture, crosses implanted in the ground)

When I even turned on the novus ordo talk show radio to ease my boredom they had a segment on forgiveness.

my boss is going to give me a second chance thanks be to God.

I'm glad it worked out for you, that you sought forgiveness and that he gave it to you.

So, the one consolation even when we sin, is that this situation gave your boss an opportunity to forgive someone else.  As a result, it was kindof a felix culpa, as it were, that the spiritual benefit of granting the forgiveness will far outweigh for him any harm done by the incident.  Even if he's not Catholic, these exercise of natural virtue can predispose one to receive the gift of faith.

So don't beat yourself up too much about the past.  What happened happened, and God allowed it to happen.  God's Providence uses even our mistakes, misjudgments, and sins for a greater good.  If He allowed this to happen to your boss, He permitted it for His good ... irrespective of your own personal dispositions at the time.